Swap the values of two variables without using third variable

Question

How many different ways can you come up with to swap the values of two variables without using any third variable ?

I use this one:

     a^=b^=a^=b

Answer 1

In Ruby:

a,b = b,a

Answer 2

Read this in my first book on C (Programming with C -- Sudhir Kaicker):

a += b;
b = a - b;
a = a - b;

Answer 3

a = a + b
b = a - b
a = a - b

There are many ways to do this. Trivially replacing +- with */ will also do it.

Answer 4

Generalising your example slightly, whenever two binary operations # and @ are inverses of each other, the following sequence will exchange the values of a and b:

a = a # b
b = a @ b
a = a @ b

Your example works because ^ (XOR) is its own inverse.

This is a neat trick. Unfortunately, some people think it makes a good interview question.

[EDIT: Thanks to mouviciel for correcting an error on the 2nd line.]

Answer 5

temp = a ; a = b ; b = temp ;

'Clever' ways are just noise, IMO.

Write code for people first, computers 2nd.

Answer 6

PHP

list($a, $b) = array($b, $a);

JavaScript 1.7

[a, b] = [b, a]

Python

a, b = b, a

Answer 7

In F#

let swap  (a,b) = (b,a);;

Answer 8

The XOR swap is a terrible, terrible thing.

Answer 9

I asked this question awhile back:

How does XOR variable swapping work?

It got some really good responses if you are curious how this is possible.

Answer 10

My Forth is way rusty, but:

. A
. B
@ A
@ B

(Does "@" write the top of the stack to a variable, or am I rustier than I thought? Anyway, this should work for all stack-based languages.)

If you're using the standard Forth idiom of treating stack entries as temporary variables, there's always:

SWAP

Answer 11

In Common Lisp:

(psetf a b b a)

Answer 12

1. Don't post any answers for interpreted languages. The interpreters are themselves whole programs with lots of variables. So, while there are no variables in your program, there are lots of variables in the program that runs your program.

EDIT: I didn't mean to offend the sensibility of those of you who use dynamic languages. They are very useful tools, but, to me, the definition of a variable is something that is in the call stack and isn't the address in memory of the instruction the program must return to after the current subroutine finishes. And those nice library functions and thingies you're used to have tons of temporary results. Those temporary results must be stored somewhere in the memory, whether you like to call it a variable or not.

1. The XOR swap is a very good one. I like it.

EDIT: The optimal implementation of the XOR swap doesn't involve any temporary results that don't have to be stored in temporary variables (not even in machine code!). This is what I meant. Using the XCHG instruction is a nice alternative.

---

Answer 13

Intel Assembly

Assuming the variables a, b are defined in .data:

mov  eax, a
xchg b, eax
mov  a, eax

Answer 14

In Lua:

a,b = b,a

You have to like it. It goes for any number of vars.

You didn't mention in what language tho :P

Answer 15

In MATLAB:

[a,b] = deal(b,a);

Answer 16

In assembly language of 8051 family , suppose X= 50 Y= 30

mov r1,#50 // r1=50 i.e. x

mov r0,#30 // r0=30 i.e. y

mov r2,r1 // r2=50, r1=50, r0=30

mov r1,r0 // r1=30, r0=30, r2=50

mov r0,r2 // r0=50, r1=30, r2=50

Hence initially X=r1=50 and Y=r0=30

after the sequence X=r1=30 and Y=r0=50

Note :- r0, r1 and r2 are registers of the 8051 microcontroller

Answer 17

Works in many languages

Works only with numbers

a = a + b - (b = a);

Answer 18

This is a simple method, logic can be implemented all the languages... Happy Sensible Coding..

$v = 'sriram';
$u = 'lakshmi';

$v .= $u;
$u = substr($v,0,(strlen($v) - strlen($u)));
$v = substr($v,(strlen($v) - strlen($u)-1), strlen($v));

echo 'u = ' . $u .'<br>';
echo 'v = ' . $v;

Answer 19

#include <iostream>
using namespace std();
void main()
{
    int a=59;
    int b=67;
    a=a+b;
    b=a-b;
    a=a-b;
    cout<<"a="<<a<<"b="<<b<<endl;
}